1. Field of the Invention
This invention relates to a silver halide photographic emulsion containing a cyan coupler which is spectrally sensitized with at least two types of sensitizing dyes having a supersensitizing effect on each other, more particularly, to a silver halide photographic emulsion which has increased spectral sensitivity in the red wavelength region.
2. Description of the Prior Art
It is well known as a technique of preparing photographic light-sensitive materials to extend the light-sensitive wavelength region of a silver halide photographic emulsion toward the longer wavelength side by adding a certain cyanine dye to the emulsion, i.e., to spectrally sensitize the emulsion. This spectral sensitivity is influenced by the chemical structure of the sensitizing dye added and by various characteristics of the emulsion, for example, the halogen composition of the silver halides, the crystal habit, the crystal system, the silver ion concentration, the hydrogen ion concentration, or the like. Moreover, the spectral sensitivity is influenced by photographic additives present in the emulsion such as a stabilizer, an anti-fogging agent, a coating aid, a flocculating agent or color couplers.
For the sensitization of a light-sensitive material in a certain spectral wavelength region, only one sensitizing dye is generally used. The combined use of sensitizing dyes often provides a spectral sensitivity lower than those obtained with the individual use of the sensitizing dyes. However, in special cases, a super-additive marked increase in spectral sensitivity is obtained when a certain sensitizing dye is used in combination with one or more other sensitizing dyes. This effect is known as supersensitization. However, strict selection is required in combining the sensitizing dye groups. Even a slight difference in chemical structure significantly affects the supersensitizing effect, and, therefore, it is difficult to predict a combination of sensitizing dyes having a supersensitizing effect based only on their chemical structural formulae.
Furthermore, the sensitizing effect on a particular emulsion can be varied by changing the nature of the emulsion. For example, the sensitizing effect can be enhanced by increasing the silver ion concentration and/or decreasing the hydrogen ion concentration. Therefore, the sensitizing effect can be increased by immersing a film coated with the spectrally sensitized emulsion in water or an aqueous solution of ammonia. The above manner of changing the sensitivity of the sensitized emulsion by increasing the silver ion concentration and/or decreasing the hydrogen ion concentration is usually referred to as hypersensitization. The shelf life of hypersensitized emulsions is generally short.
When supersensitization is applied to a silver halide photographic emulsion, the sensitizing dyes used must not adversely interact with photographic additives other than the sensitizing dyes and stable photographic properties must be maintained during storage of the light-sensitive material. Another requirement for the sensitizing dyes used is that no residual color remain on the processed light-sensitive material due to the sensitizing dyes. It is particularly required that no residual color remain in the case of short-time processing (usually taking several seconds to several tens of seconds) such as rapid processing.
In order to obtain excellent color reproducibility in a color light-sensitive material, it is not desired that the red-sensitive layer possess high sensitivity for too long a wavelength, for example, has a maximum sensitization at a wavelength longer than 660 nm, or that it possess sensitivity only in too short a wavelength region, for example, has a maximum sensitization at a wavelength shorter than 580 nm. Unfortunately, it is difficult with spectral sensitization techniques to selectively increase sensitivity in the wavelength region ranging from about 580 nm to about 630 nm, and, therefore, to solve this problem is one important object in the art.